[ConstraintElim] Update existing constraint system in place (NFC).

This patch breaks up the solving step into 2 phases:

1. Collect all rows where the variable to eliminate is != 0 and remove
   it from the original system.
2. Process all collect rows to build new set of constraints, add them to
   the original system.

This is much more efficient for excessive cases, as this avoids a large
number of moves to the new system. This reduces the time spent in
ConstraintElimination for the test case shared in D135915 from ~3s to
0.6s.